Wines, beers, fruit juices, water and the protein containing streams from bioreactors and other such liquids need to be filtered to remove impurities such as sugar crystals from wine; or cell debris and other cellular components from the bioreactor stream, or yeast from beer, or grape, other fruit skins and other debris found in raw and even partially purified liquids.
Traditionally, this has been done with loose diatomaceous earth that is added to the liquid and allowed to settle. More advanced technology has become the various lenticular or plate and frame products that are formed of a cellulosic fiber material and various fillers such as diatomaceous earth and various charged polymers. These filter cartridges are normal flow devices, meaning that liquid is directed at them and the liquid flows through them to an outlet while impurities remain trapped in the pad(s). These filter cartridges are arranged in a housing which may contain one or more, generally several of these filter cartridges either in a frame in a plate and frame device or on a central rod that has a series of holes leading to the outlet within the rod in a lenticular pad device. Liquid enters through an inlet generally at the bottom side of the housing and flows around and through the filter(s) to the openings in the rod and out the bottom of the housing through an outlet. The pad filter cartridges become easily clogged with impurities that limit their use in production runs. This often requires the use of two or more identical filter systems that are run sequentially during a filtration operation to ensure adequate capacity.
More recently, users have begun to look at TFF (tangential flow filtration) devices for such filtration applications. The TFF systems utilize the tangential flow across the face of the membrane to constantly sweep the membrane surface clean of materials that could block or plug the membrane leading to a decline in filter efficiency and life time. These devices can be made of flat or hollow fiber ceramic filters or flat, hollow fiber or spiral wound polymeric filters.
While helpful in increasing the life of the filter system, such TFF systems have drawbacks including cost both of the equipment and to run it as well as high hold-up volumes and potential injury/degradation to the liquid due to numerous recirculations through the pump(s) of the system.
TFF filter systems are more capital intensive than pad filters and require extensive specialized equipment including special filters, holders, pumps, retentate lines, etc. that are expensive to purchase. Additionally, the process using this type of filter is different and requires special training and dedicated capital equipment to do so. Moreover, the system uses 10 to 20 times the energy of a pad system for the pumping to create adequate recirculation of the fluid to create and maintain the TFF flow. Likewise, due to the extensive plumbing used in such systems, there is a large holdup volume in the system after filtration is completed. The fluid that has not been filtered and is in the tank, retentate lines, pumps, etc. is discarded after the filtration process. Often this is measured in 10 s of gallons of liquid that is lost and can amount up to 10% (depending of the hold up capacity of the system and the length and number of repeat cycles of filtration) of the starting volume leading to lower overall yield of product. Lastly, many liquids are sensitive to shear and other similar forces imposed upon them in TFF devices and can degrade or injure the product especially wine.
Ceramic filters work but have the additional disadvantage of being susceptible to breakage under a variety of conditions which for the most part can't identified or quantified. An alternative to TFF ceramic filters is now a plastic or polymeric hollow fiber system with low or limited cross flow so as to reduce the number of cycles that the liquid is recirculated. While stating that the quality of the liquid is improved by reducing the recirculation, there are still the drawbacks of expense and large hold up volume. However and more importantly, the low crossflow is accomplished by increasing the surface area of the membranes used, often by at least 2× that of a traditional TFF system, again increasing cost and holdup volume. Additionally, TFF polymeric membranes are relatively weak in strength and are susceptible to rupture if too much pressure or shear is applied to them. Additionally, cleaning such filters is difficult, costly and time consuming making the cost of such filtration much more expensive
Besides, TFF is in general very difficult to apply to cold filtration such as may be used with wine, beer, and fruit juice applications as higher viscosity of the liquid due to the lower temperature leads to lower flow rates and therefore bigger systems making them difficult to be economically viable. Another disadvantage in using polymeric TFF filters in cold wine or fruit juice applications which often include the filtration of crystals is that the polymeric membranes are not compatible with the “scratching” effect of the crystals of sugar in cold filtration causing wear on the filters and the production of particles into the fluid.
What is needed is a system for filtration of high solids systems that is simple to set up, run and is less expensive than the TFF systems to purchase and operate and which increase yields with minimal filtration passes through the system to minimize degradation.